Diabetes mellitus (DM) has been found to be associated with depressed left ventricular (LV) function. Right ventricular (RV) function in DM patients, however, has not been well studied. The goal of this study was to evaluate the occurrence of LV and RV dysfunction in patients with DM. A series of 157 patients underwent simultaneous measurement of LV ejection fraction (LVEF) and RV ejection fraction (RVEF). Four of 26 DM patients had RVEF <30% (15.4%) vs 4 of 126 controls (3.2%) (P=.01). Eleven of 27 (40.7%) patients with DM had LVEF <30% vs 9 of 128 controls (7%) (P<.0001). Using multivariate analysis, DM remained independently associated with severely decreased biventricular function (RVEF <30%; odds ratio, 5.7; confidence interval, 1.3–25.4 [P=.02] and LVEF <30%; odds ratio, 12.9; confidence interval, 3.8–43.7 [P<.0001]). These results suggest that diabetic cardiomyopathy involves both ventricles as an independent pathologic process.